Large scale containers are typically used to transport goods internationally and domestically. Quantities of such containers are loaded and unloaded at ports on an ongoing basis. Due to the large quantity of containers that are received at ports, port inspectors may not be able to open the containers to inspect their contents. This can pose a security risk.
To address the security risk introduced by an inability to open and inspect the contents of shipping containers, cargo inspection devices have been developed that scan the insides of the containers without requiring inspectors to open the containers. Conventional cargo inspection devices perform radioscopic examination of shipping containers using an X-ray beam or gamma beam that can penetrate the container to identify its contents. For inspecting filled shipping containers, a cargo inspection device that produces X-ray beams using an accelerator is typically used because of the high energy output (and therefore greater penetration) that it provides.
Typically, the linear accelerators used in cargo inspection systems are configured to produce a single energy X-ray beam. A detector receives the single energy X-ray beam that has penetrated the shipping container without being absorbed or scattered, and produces an image of the contents of the shipping container. The image can be displayed to an inspector who can perform visual inspection of the contents.
Some cargo inspection devices use dual energy linear accelerators that are configured to emit two different energy level X-ray beams. With a dual energy X-ray inspection system, materials can be discriminated radiographically by alternately irradiating an object with X-ray beams of two different energies. Dual energy X-ray inspection systems can determine a material's mass absorption coefficient, and therefore the effective atomic (Z) number of the material. Differentiation is achieved by comparing the attenuation ratio obtained from irradiating the container with low-energy X-rays to the attenuation ratio obtained from irradiating the container with high-energy X-rays. Discrimination is possible because different materials have different degrees of attenuation for high-energy X-rays and low-energy X-rays, and that allows identification of low-Z-number materials (such as but not limited to organic materials), medium-Z-number materials (such as but not limited to transition metals), and high-Z-number materials (such as but not limited to radioactive materials) in the container. Such systems can therefore provide an image of the cargo contents and identify the materials that the cargo contents are comprised of.
The ability of dual energy X-ray inspection systems to detect the Z number of materials being scanned enables such inspection systems to automatically detect the different materials in a container, including radioactive materials and contraband such as but not limited to cocaine and marijuana. However, conventional dual energy X-ray inspection systems use a standing wave linear accelerator that is vulnerable to frequency and power jitter and temperature fluctuations, causing the beam energy from the linear accelerator to be unstable when operated to accelerate electrons to a low energy. The energy jitter and fluctuations can create image artifacts, which cause an improper Z number of a scanned material to be identified. This can cause false positives (in which a targeted material is identified even though no targeted material is present) and false negatives (in which a targeted material is not identified even though targeted material is present).